Ground heave is a well known phenomenon, arising particularly but not exclusively in clay soils, in which the substrate expands (e.g. in the event of prolonged rain following a long period without rain, or after removal of a tree from adjacent a structure which upsets the substrate water balance) exerting large pressures on any structure built onto or into the substrate leading to cracking of the structure foundations and walls and--in the extreme--to the complete failure of the structure.
Clearly it would be possible to avoid this problem by ensuring that a void is provided beneath the lowermost part of the structure as it is built into which void the substrate may move--if subject to heave--without affecting the building thereabove. Such a solution is possible only if the lowermost part of the building is preformed since if it is being built "on-site" it needs to be supported--at least until it is sufficiently stable to stand on its own above the substrate.
To overcome this problem and permit structures to be built on substrates in which heave is likely to occur it has been proposed to provide a compressible volume between the parts of the structure in contact with the substrate and the substrate itself (notably the ground beams and ground slabs used in the structure).
One such known proposal is to provide a compressible foamed plastics material (e.g. expanded polystyrene) layers between the substrate upon which the structure is being built and the ground beams and ground slabs of the structure. Such a solution adds greatly to the safety of a structure, when the substrate on which it is built heaves, by reducing by partial absorption the stress transmitted to the structure. However, the compressibility of the plastics foams known to us is limited and the material always transmits a certain amount of loading to the structure. As a result the thickness of the foamed plastics layers required are much greater than would be needed if a complete void were provided beneath the structure (up to 2.5 times the thickness). This exacerbates another disadvantage--that of the foamed plastics layer compressing under the weight of concrete as it is poured.
Another known proposal provides a sandwich support arrangement having wood or fiber boarding mounted on either side of a central, fibrous paper-like honeycomb. When dry the central honeycomb section of the support arrangement will support the weight of wet concrete as it is poured, but when wet its ability so to do is considerably reduced. Such an arrangement offers advantages over the foamed plastics layers proposal and the support need be only 10-15 mm deeper than would have been a complete void.
A disadvantage of this proposal, however, is the need to keep the central honeycomb section of the arrangement dry for it to retain its strength whilst it is supporting, for example, concrete being poured to form a ground beam or a reinforced concrete ground slab. Thus when using arrangements embodying this proposal it is now becoming a common requirement completely to cover the support arrangement with a water impermeable sheet--e.g. polythene--to prevent the central honeycomb section of the arrangement collapsing under the weight of the concrete being poured (or even, prior to the concrete being poured, by the weight of bars placed on it to reinforce the concrete) following a shower of rain--or even from the effects of moisture in the concrete itself.
Further disadvantages of such a sandwich support arrangement are that, under certain conditions, it can (like other wood and certain cellulose products) biodegrade to form methane gas, which is dangerous (see `New Civil Engineer` of Apr. 11, 1991), and that it can harbour and promote infestation and/or dry rot.
Objects of the invention include the provision of methods of building structures and of apparatus and arrangements for use in such methods which overcome or at least alleviate the above-mentioned and/or other problems or disadvantages of the prior art.